Conductive probe level control circuits are well known, as shown by U.S. Pat. No. 4,019,067 issued Apr. 19, 1977 to R. Gladstone. In this circuit, a low voltage AC is impressed between the probe or sensing electrode and the containing vessel for the liquid. The sensing output acts on the control electrode of an SCR to activate a relay or the like.
Guard ring or guard shield electrodes for controlling leakage paths are known from U.S. patents such as U.S. Pat. No. 3,119,266 issued Jan. 28, 1964 to D. Atkinson and U.S. Pat. No. 4,027,172 issued May 31, 1977 to W. Hamelink. In the first of these two patents, the guard ring or shield is driven in phase with the probe voltage by an amplifier in the probe output circuit to raise the effective impedance of the leakage resistance counteracting the leakage effects.
In the second of the two cited references, U.S. Pat. No. 4,027,172, direct current leakage paths are placed in shunt of the probe. Excessive current leakage across either of two leakage paths (guard shield to probe or guard shield to tank body) shunts out the probe path and causes an output signal to deactivate the apparatus.